Between those two dates there was an extremely powerful campaign on the web attacking his role, which, as a former speech writer for the George W Bush administration, was envisaged by the Guardian as someone who would give insight into the Republican campaign.

The attacks were led chiefly by the Electronic Intifada website and heavily supported by the Palestinian Solidarity Campaign and its members. The Guardian readers' editor's office had received almost 200 complaints by the end of last week. In the first 24 hours the complaints turned on a tweet posted by Treviño on 25 June 2011: "Dear IDF: If you end up shooting any Americans on the new Gaza flotilla – well, most Americans are cool with that. Including me." This was seen by the complainants as an incitement to shoot Americans taking part in a flotilla of boats that planned to break the blockade of Gaza in 2011. An attempt in 2010 to break the blockade ended with the interception of the boats by the Israelis and the deaths of nine protesters.

While the remit of the Guardian's readers' editor is wide ranging, I did not feel that a tweet from a private account before he was contracted to the Guardian fell within it.

However, as a result of the first flush of complaints, Treviño wrote a blogpost for the Guardian's US site on 16 August, which he described as a clarification. In it he recognised that this particular tweet might lead people to believe he was inciting the IDF to shoot Americans but he strongly denied this – three times – in the article. He wrote: "I urged no such thing. I intended no such thing. But sufficient numbers believe I did, and in cases of widespread misapprehension of meaning, the fault always lies with the writer." He also apologised for giving "the impression that I welcome killing".

This triggered a further complaint from Ali Abunimah, co-founder of the Electronic Intifada, that this latest Treviño article was inaccurate because there was ample evidence from other tweets to show that Treviño did mean to encourage the shooting of Americans in that flotilla. As this complaint relates to a current piece of journalism on the Guardian website, this falls within the readers' editor's remit.

I have reviewed Abunimah's complaint. While I think it likely that a reasonable person might well believe this was the intent of the tweet, I don't think it is possible to make an objective finding of inaccuracy about his denial. The tweet states clearly that he would be "cool" ie relaxed about them being shot. In the article he denies absolutely that he meant this to be taken as an encouragement to the IDF to kill Americans. I believe the complaint would require a judgment on Treviño's sincerity: a matter of opinion, not a decision based on factual accuracy.

There was a second complaint on Thursday 23 August received by senior editorial staff in the US and referred to the readers' editor. This concerns another blogpost Treviño had written as a contributor to the Guardian's US site – before he was on contract – on 28 February 2011 about a Republican congressman's inquiry into Islamic radicalisation, which quoted the Malaysian prime minister Najib Razak.

Until shortly before this blogpost, the author had been a consultant for an agency retained by Malaysian business interests and ran a website called Malaysia Matters, which should have led to a footnote disclosing the relationship. Failure to declare a potential conflict of interest is a breach of the Guardian's editorial code. This relationship will now be footnoted on the blogpost and, as the article was not in print, a correction included in the Guardian's online corrections and clarifications. A Guardian spokesperson said that following this disclosure, both Treviño and the Guardian agreed to end the contract.

This has been a bruising 10 days for the Guardian that could have been handled better. Some of the issues were already in the public domain and could have been addressed earlier.

Janine Gibson, editor-in-chief of Guardian US, said: "We didn't know about the tweet. Trevino has a combative and prolific Twitter presence – there are something like 81,000 tweets. When it was drawn to our attention, we too were horrified and asked for an explanation. He assured us that it was a casual and horrendous use of language on social media.

"Sharing that explanation with our readers, while never going to change anyone's mind, was the most open way, we felt, of acknowledging what was a reprehensible tweet and allowing us to focus on what we had engaged him to do, which was, on the eve of the Republican convention and in the middle of an already vicious and highly partisan election campaign, explain and analyse the politics of the US Republican party.

"This has been an eye-opening week. We knew that there are dangers inherent in attempting to be fair minded and allow our opponents as well as our friends a voice and we have learned several lessons. But I hope we will continue to try and find ways to engage with honestly held philosophies and opinions."